A “gel” can be defined as a colloid which is in a form that is more solid than a sol. A “hydrogel” is a gel in which the liquid or dispersion medium is water. Hydrogels can be formed from virtually any substance that is capable of forming stable colloids with water. For example, silica hydrogels are well known and have been utilized in many applications. Hydrogels also can be formed from polymeric substances, such as hydrophilic monomers or polymers.
One of the most common methods for producing hydrogels from hydrophilic monomers or polymers utilizes an aqueous solution of the hydrophilic monomer or polymer. In particular, the process begins with the preparation of a dilute aqueous solution of the hydrophilic monomer or polymer. The dilute solution is then irradiated or otherwise treated (e.g., with a chemical crosslinking agent) in order to form cross-links between the individual monomer or polymer molecules present in the solution. As more and more crosslinks are formed between the molecules, the hydrophilic monomer or polymer begins to form a network of polymer molecules that grows until the aqueous solution gels. The resulting hydrogel contains a three dimensional matrix of cross-linked polymer molecules in which the water from the aqueous solution is encapsulated.
Hydrogels made by the aforementioned process are used in many applications. However, the limitations inherent in the process impact the properties of the hydrogels, which prevents them from being used in many applications. For example, it is often difficult to produce an initial aqueous solution of the hydrophilic monomer or polymer having a concentration high enough to produce a hydrogel having an extensive matrix of crosslinked polymer molecules and any significant mechanical strength (e.g., compressive strength and/or tensile strength). Indeed, the extent of the polymer network and mechanical properties of the hydrogel can suffer even more when the hydrogel is made from medium to high-molecular weight hydrophilic polymers, which tend to have relatively low water solubilities that prevent the use of aqueous dispersions containing any significant amount of the hydrophilic polymer. The relatively weak mechanical properties exhibited by hydrogels made by this process severely hamper the usefulness of such hydrogels in applications that may subject the hydrogel to even moderate mechanical stresses, such as tensile and/or compressive stresses.
Furthermore, insofar as the process requires a uniform aqueous solution of the hydrophilic monomer or polymer, it is difficult to produce a hydrogel that also contains a water-insoluble material, such as a polymer, uniformly dispersed in the hydrogel. More specifically, it is often difficult to maintain a uniform dispersion of the water-insoluble material in the initial aqueous solution of the hydrophilic monomer or polymer such that the water-insoluble material is uniformly distributed in the hydrogel after the monomer or polymer is crosslinked to produce the hydrogel. Utilizing the aforementioned process, it is also often difficult to control the final water content of the hydrogels produced by the process. In particular, the final water content of hydrogels produced by the aforementioned process is largely determined by the chemical composition of the hydrogel (i.e., the hydrophilic polymer used to make the hydrogel) and the degree to which the hydrophilic polymer molecules are cross-linked. Therefore, the amount of water contained in the hydrogel will be determined, at least in part, by the particular hydrophilic polymer used to make the hydrogel as well as the degree of cross-linking required to produce a hydrogel having the desired characteristics, and not solely by the desired final water content of the hydrogel.
A need therefore remains for hydrogel compositions exhibiting enhanced mechanical properties relative to hydrogel compositions produced by typical methods. A need also remains for methods of making such hydrogel compositions exhibiting enhanced mechanical properties. The invention provides such a hydrogel composition and methods for making the same. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.